It is known practice to produce (meth)acrylic esters from an esterification reaction between an alcohol and a (meth)acrylic acid. This reaction is an equilibrated catalyzed reaction with generation of water. It is also accompanied by side reactions producing impurities.
It is necessary to eliminate the water produced in order to shift the equilibrium, and it is necessary to recycle unreacted reagents (alcohol and acid) and also to eliminate impurities, in particular lighter compounds than the ester and heavier compounds than the ester, so as to obtain a product which meets commercial specifications.
To this end, a set of treatments of the reaction mixture is generally carried out, by means of distillations and/or extractions, or settling out operations, which set of treatments is both relatively complex to carry out, especially due to the presence of azeotropic mixtures, and costly in terms of energy.
The reaction mixture contains the desired ester, water, unreacted acid and alcohol, “light” by-products having a boiling point lower than that of the ester, and “heavy” by-products, that is to say having a boiling point higher than that of the ester. The purification sequence applied to the reaction mixture generates various streams, the composition of which varies depending on the apolar nature of the alcohol and of the ester, that is to say according to the length of the alkyl chain of the alcohol used. These streams share the common feature of containing water resulting from the reaction and/or the extraction steps.
In the process for producing (meth)acrylic ester described in patent application FR 2 980 475 in the applicant's name, modules for membrane separation dehydration are used to dehydrate streams comprising (meth)acrylic ester and unreacted alcohol with a view to effectively eliminating water and to produce good selectivity for (meth)acrylic ester. These modules are especially applied at the reaction step or at the unreacted alcohol recycling for a well-defined range of concentrations of the various compounds, and mainly in the case of producing C1-C4 alkyl (meth)acrylates such as methyl acrylate, ethyl acrylate or butyl acrylate.
Surprisingly, the applicant has now observed that, in the case of producing C4-C10 alkyl (meth)acrylates, it is possible to apply a step of membrane separation dehydration to aqueous streams outside the concentration ranges described in document FR 2 980 475. This leads to the effective elimination of the water present in the streams intended to be purified and/or recycled, which are separate from the treated aqueous streams in the process of the abovementioned document, and thus avoids the formation of a loop of water which is harmful to the productivity and energy consumption of the process. In particular, the dehydration step is advantageously applied upstream of the finishing column for the desired ester, or at the top of the column for separating the light compounds, or on the aqueous phase after settling out of the crude reaction mixture.